Austenitic-ferritic stainless steel alloys are well-known since the 1930s and 1940s and are essentially based on compositions with a high chromium, nickel and molybdenum content. Reference could in this sense be made to U.S. Pat. No. 2,432,616, applied for in 1945 by FRANKS et al., which relates to alloys with a high mechanical strength at high temperatures, one of the alloys disclosed consisting of not more than 0.35% carbon, not more than 2.0% manganese, not more than 1.0% silicon, 2-40% nickel, 10-30% chromium, 1-5% molybdenum, not more than 0.25% nitrogen, 0.5-5-0% tungsten and 0.1-0.7% boron.
Steels of this kind have evolved over the years, changing their structure to adapt to the various needs required of materials as regards mechanical strength, corrosion resistance, good structural stability and the like. In this sense, duplex and superduplex steels are well-known and used. Duplex (ferritic-austenitic) steels have a composition with a chromium percentage of 22%, 5% nickel, 3% molybdenum and 0.17% nitrogen, which gives them a good corrosion resistance in marine and similar environments. The composition of superduplex steels, on the other hand, is 25% chromium, 6.8% nickel, 3.7% molybdenum and 0.27% nitrogen, to reach a rate of pitting resistance in excess of 40. The increased percentages of chromium, nickel and molybdenum used over duplex steels provides such enhanced properties as mechanical strength and corrosion resistance.
It is moreover normal for the standard compositions of duplex and superduplex alloys to include other elements, albeit in smaller quantities, such as manganese, sulphur, silicon, copper, tungsten, magnesium, aluminium, rare earths and the like.
Reference can in this sense be made to a number of duplex or superduplex Patents in which some of the components vary over the usual standards in order to enhance a particular characteristic thereof, generally associated with a specific application of the resultant steels, namely for instance:
GB Patent 2 160 221 to NIPPON KOKAN K.K. relates to a ferritic-austenitic stainless steel for making seamless tubes with an enhanced impact toughness, therefore disclosing the addition of soluble aluminium in a quantity not exceeding 0.02%. The other components lie approximately within the usual ranges of ferritic-austenitic steels of this kind.
U.S. Pat. No. 4,604,887 to OHTSUBO et al. relates to a manufacturing method which avoids surface defects, along with flaws at the head and tail of the tubes, due to special rolling conditions and a special chemical composition, which is essentially distinguished from other duplex steels by a high copper content and the addition of small quantities of aluminium.
European Patent 0 339 004 to SANDVIK AG relates to an austenitic-ferritic stainless steel which includes tungsten, vanadium and cerium in the alloy to provide the resultant steel with high resistance in chloride ion environments and preferably applicable for manufacturing medical implants. A steels of similar characteristics is also described in European Patent 0 220 141 to SANTRADE LTD.
GB Patent 2 203 680 to NIPPON YAKIN KOGYO CO. LTD. relates to a process for continuously making ferritic-austenitic stainless steel, to which end considerable quantities of tungsten and vanadium are added, along with very small quantities of boron, the other elements lying within the usual percentages.
European Patent no. 0 545 753 to SUMITOMO METAL INDUSTRIES LTD. relates to a superduplex steel of high mechanical strength and corrosion resistance, albeit with a very reduced tendency towards intermetallic precipitation, the main characteristic of the alloy disclosed being the addition of higher than usual tungsten percentages, for the quantities of tungsten and chromium may thus be reduced and consequently so may the negative effects of these elements as regards the precipitation of the intermetallic compounds be minimised. Small quantities of other elements such as vanadium, calcium, magnesium, boron or rare earths are also added to improve the hot-ductility of the resulting steel.
European Patent no. 0 594 935 to CENTRO SVLUPPO MATERIALI S.p.A. relates to a stainless steel alloy and the respective process for obtaining seamless tubes, based on the aforesaid alloy, useful in acid environments, namely for instance in oil exploration. The alloy subject of this invention is essentially characterised by the addition of greater than usual tungsten and copper percentages, in order to improve the mechanical and corrosion resistance properties.
European Patent no. 0 566 814 to FORONI S.p.A. relates to a superduplex steel with a high corrosion resistance in an acid or alkaline environment, for which purpose greater than usual quantities of copper and tungsten are added.